An array antenna radio communication apparatus is a radio communication apparatus that comprises a plurality of antennas and is capable of setting directivity freely by making an adjustment of each of amplitude and a phase to signals received via the respective antennas. The adjustments of amplitude and the phase to a received signal is carried out by multiplying the received signal by weight coefficient (hereinafter referred to as “weight”).
The array antenna radio communication apparatus adjusts weights to multiply, making it possible to receive only a signal incoming from a desired direction intensively. It is thereby possible for the array antenna radio communication apparatus to maintain a reception SIR (Signal to Interface Ratio) on the signal incoming from the desired direction.
The conventional array antenna radio communication apparatus, however, has the following problems when it is applied to a mobile communication base station apparatus (hereinafter referred to “base station”) used in a CDMA radio communication system.
More specifically, in the CDMA radio communication system, communication between the base station and each mobile communication terminal apparatus (hereinafter referred to as “mobile station”) is performed in such a way that a different spread code is assigned to each mobile station and the same frequency band is used at each mobile station.
In the case where a mobile station that transmits signals having high reception power in the base station and a mobile station that transmits signals having low reception power in the base station exist, the signals having high reception power cause interference with signals having low reception power since communications are performed using the same frequency band. For this reason, the base station cannot maintain the reception SIR of signals having low reception power high, with the result that the base station cannot find out the direction where the mobile station that transmits signals having low reception power exists.
Accordingly, this causes a problem in which the base station can form directivity to the mobile station that transmits signals having high reception power but cannot form directivity to the mobile station that transmits signals having low reception power.
Further, in the case where the base station cannot form directivity to the mobile station that transmits signals having low reception power, the base station transmits signals in all directions when transmitting signals to the mobile station that transmits signals having low reception power. This causes a problem in which the signals, which are sent to the mobile station that transmits signals with low reception power, result in interference signals with other mobile stations.
Furthermore, in the base station that performs weight control using an algorithm of MMSE (Minimum Mean Square Error) standard, the signals with low reception power do not occur interference with the signals with high reception power in some instances. In this case, the base station can maintain reception SIR of signals with high reception power sufficiently high without forming directivity to the mobile station that transmits signals with high reception power. Namely, the base station does not form directivity to the mobile station that transmits signals with high reception power. In the case where the base station does not form directivity to the mobile station that transmits signals with high reception power, the base station transmits signals in all directions when transmitting signals to the mobile station that transmits signals with high reception power. Hence, this causes a problem in which the signals, which are sent to the mobile station that transmits signals with high reception power, result in interference signals with other mobile stations.